1. Field of the Invention
The invention is related to diode lasers, and, more particularly, to separating diode lasers from wafers.
2. Discussion of the Prior Art
Coherent light-emitting diodes having a GaAs-(Al,Ga)As double heterostructure, such as described in "Semiconductor Lasers and Heterojunction LED's" by H. Kressel and J. K. Butler, Academic Press, New York, 1977, are known to be efficient light sources for optical communication systems.
As is well-known, such diode lasers comprise layers of GaAs and (Al,Ga)As on an n-GaAs substrate. The final layer is a cap layer of p-GaAs. Metallized stripes, parallel to the intended direction of lasing, are deposited on a p-side of the wafer. Gold contact pads, somewhat smaller in area than the intended size of the diode laser, are deposited on the n-side of the wafer. The stripes and pads are for subsequent connection to an external electrical source.
The wafer is then cut into two mutually orthogonal directions to form the individual diodes. First, the wafer is cut perpendicular to the intended lasing facets into bars of diodes, then the bars of diodes, following passivation of lasing facets, are cut into individual diodes.
Cutting of the wafer into bars is generally accomplished by cleaving the wafer through the substrate side, using an instrument such as a razor blade, knife, scapel blade or the like. Control over length of the diode laser is consequently poor, and variation of diode laser length is great, with the result that longitudinal mode distribution and threshold current vary considerably from one diode laser to the next. Further, the gold contact pads must be kept thin in order to permit reasonably clean cleaving. Also, the thickness of the substrate is constrained in order to promote better cleaving. This limits useful wafer thicknesses to about 3 to 5 mils. Yet, such thin wafers are highly susceptible to breaking during handling. Finally, striations generated by the mechanical cleaving, if across the active lasing region, affect dvice yield, since such devices are consequently prone to degradation.
In a related patent application, Ser. No. 951,074, an etch method of cleaving semiconductor diode wafers is disclosed and claimed. The wafer is first separated into bars of diodes by a process which comprises (a) forming an array of exposed lines on the n-side by photolithography to define the lasing ends of the diodes, (b) etching through the exposed metallized portion to expose portions of the underlying n-GaAs, (c) etching into the n-GaAs substrate with a V-groove etchant to a distance of about 1 to 2 mils less than the total thickness of the wafer and (d) mechanically cleaving into bars of diodes. The disclosed technique is useful for wafers having a total thickness of about 3 to 5 mils and long cavity lengths (about 10 mils or greater). For thicker wafers, such as about 6 to 10 mils in thickness, however, etching with a V-groove etchant consumes too much of the surface of the n-substrate, thus making contact to the n-side difficult. On the other hand, the disclosed technique does result in improved length definition and uniformity, in increased device yields and in reduced striations on lasing facets, and thus represents a substantial improvement over prior art techniques. A need, however, remains in the art for a method of cleaving semiconductor diode lasers from relatively thick wafers.